Retroreflective sheeting has the ability to redirect incident light towards its originating source. This advantageous property has led to the widespread use of retroreflective sheeting on a variety of articles. Typical examples of retroreflective sheeting are microsphere-based sheeting and cube-corner sheeting.
Microsphere-based sheeting, sometimes referred to as "beaded" sheeting, is well known in the art and employs a multitude of glass or ceramic microspheres, typically at least partially embedded in a binder layer and having associated specular or diffuse reflecting materials (e.g., pigment particles, metal flakes, or vapor coats) to retroreflect incident light. Examples of such retroreflectors are disclosed in, for example, U.S. Pat. Nos. 3,190,178 (McKenzie), 4,025,159 (McGrath), 5,064,272 (Bailey et al.), and 5,066,098 (Kult).
Structured retroreflectors typically comprise a sheet having a generally planar front surface and an array of structured retroreflecting elements protruding from the back surface. One variety of structured retroreflectors are cube-corner retroreflectors. Cube-corner reflecting elements comprise generally trihedral structures that have three approximately mutually perpendicular lateral faces meeting in a single corner. In use, the retroreflector is arranged with the front surface disposed generally toward the anticipated location of intended observers. Light incident to the front surface enters the sheet, passes through the body of the sheet to be internally reflected by the faces of the structured elements so as to exit the front surface in a direction substantially toward the light source (i.e., it is retroreflected).
The light rays are typically reflected at the faces of the structured elements due to either total internal reflection (TIR), or due to specular reflective coatings such as a vapor-deposited aluminum film. Reflectors relying on total internal reflection require an interface between the faces and a material, typically air, having a lower index of refraction. Examples of cube-corner structured retroreflective sheetings are disclosed in U.S. Pat. Nos. 3,712,706 (Stamm), 4,025,159 (McGrath), 4,202,600 (Burke et al.), 4,243,618 (Van Arnam), 4,349,598 (White), 4,576,850 (Martens), 4,588,258 (Hoopman), 4,775,219 (Appeldom et al.), 4,895,428 (Nelson et al.), 5,272,562 (Coderre), and 5,450,235 (Smith et al.), as well as in PCT Publication Nos. WO 95/11464 (Benson et al.) and WO 95/11469 (Benson et al.). Typically, structured retroreflective sheetings exhibit a retroreflective brightness (i.e., a coefficient of retroreflection) of greater than about 50 candela/lux/square meter.
In applications in which the structured retroreflective sheeting is likely to be exposed to moisture (e.g., outdoors or in high humidity), the structured retroreflective elements can be encapsulated with a conformable sealing film, as disclosed, for example, U.S. Pat. Nos. 4,025,159 (McGrath), 5,117,304 (Huang), and 5,272,562 (Coderre). Conventional sealing films may be single or multi-layer thermoplastic or thermoplastic/thermoset films that are attached to the structured surface. The sealing film maintains an air interface around the structured elements to maintain retroreflectivity due to the lower refractive index. The sealing film also protects the surfaces from degradation caused by environmental exposure.
The sealing film is typically attached to the structured film with the application of heat and pressure using an embossing tool on the sealing film to create a cellular pattern (i.e., cells). These contact areas between the sealing film and the tips of the structured elements do not retroreflect. In addition, the heat used to bond the sealing film to the structured film can also distort the structured elements near the bonds, reducing their ability to retroreflect light.